Apparatus for beam splitting

ABSTRACT

An apparatus for splitting a light beam into two sub-beams is formed of a stationary light source for generating a light beam, of a first polarization converter and of a deflection unit rotating around the optical axis in which two sub-beams are generated from the light beam. The deflection unit contains a polarization beam splitter, a following, second polarization converter and a reflector that is preferably designed as a prism. An automatic correction of positional offsets of the sub-beams is implemented in the deflection unit since the polarization beam splitter or the deflection unit is tilted and/or the lenses are shifted with adjustment drives. The positional offsets of the sub-beams are measured in their focus plane, and corresponding correction values are transmitted with a transformer to the adjustment drives in the rotating deflection unit.

BACKGROUND OF THE INVENTION Field of the Invention

The invention is in the field of reproduction technology and is directedto an apparatus for splitting a light beam into two sub-beams.

Such an apparatus for beam splitting can be employed, for example, in alight beam scanner unit for originals scanner devices or recordingdevices.

In an originals scanner device, also referred to as an input scanner,the light beam sweeps the original to be scanned point-by-point andline-by-line, and the scan light reflected from or allowed to pass bythe original is converted into an image signal in an optoelectronictransducer.

In a recording device, also referred to as a recorder, exposer or outputscanner, the light beam is intensity-modulated by an image signal, andthe intensity-modulated light beam is conducted over a light-sensitiverecording material point-by-point and line-by-line.

In a scanner or recorder device of the flatbed type, the holder for theoriginal or, the recording material is a flat surface moved relative tothe light beam scanner unit that the light beam sweeps point-by-pointand line-by-line.

In a scanner or recorder device of the inside drum type, the holder forthe original or, the recording material is designed as a stationarytrough shaped like a cylinder segment. The light beam scanner unit movesparallel to the longitudinal axis of the trough, and the light. beam isradially conducted over the trough point-by-point and line-by-lineperpendicular to the longitudinal axis.

DE-A-41 28 468 already discloses a light beam scanner unit with anapparatus for splitting a light beam into two sub-beams in a recordingdevice of the inside drum type. The beam splitter apparatus comprisesessentially a stationary light source and a deflection unit that turnsaround an optical axis. The deflection unit is composed of apolarization beam splitter, a polarization converter and a plane mirror.

The polarized light coming from the stationary light source is dividedinto two sub-beams in the rotating deflection unit by the polarizationbeam splitter, these two sub-beams being intensity-modulated by an imagesignal. The two intensity-modulated sub-beams emerge radially offsetfrom the deflection unit, are focussed onto a recording material fixedin an exposure trough and expose the recording material point-by-pointand line-by-line. Two lines on the recording material are exposed withthe two intensity-modulated sub-beams per revolution, a high recordingspeed being achieved as a result thereof.

In order to also achieve a high recording quality, the two sub-beamsdare not exhibit any positional offset in the recording planeperpendicular to the line direction compared to a rated position inwhich the two sub-beams are congruent. As a result of such a positionaloffset, namely, the two lines exposed on the recording material wouldnot proceed equidistantly from one another, and disturbing fluctuationsin tonal value that considerably diminish the recording quality wouldresult.

The known beam splitter apparatus has the disadvantage that the lightbeam generated in the stationary light source must be very preciselyadjusted in the direction of the optical axis or, of the rotational axisof the deflection unit so that, after half a revolution of thedeflection unit, the sub-beams are incident on a line in the recordingplane perpendicular to the line direction without positional offset.When the light beam in the known beam splitter apparatus is not alignedexactly, the two sub-beams, however, exhibit an oppositely directedpositional offset relative to the rated position, i.e. the beampositions lies to the left and right of the rated position, as a resultwhereof a relatively big positional error arises. An exact adjustment,however, is involved and is lost over time, so that a re-adjustment ispotentially required. Disturbing positional offsets of the sub-beamsalso arise when the polarization beam splitter and the plane mirror inthe deflection unit are not aligned exactly relative to one another dueto manufacturing tolerances. For compensation of errors that have arisendue to an imprecise alignment of the optical components, the known beamsplitter apparatus comprises, for example, an adjustable opticaladjustment mechanism in the form of a camera wedge arranged in the beampath between light source and deflection unit. Such an adjustment,however, is relatively involved and imprecise, since it can only becarried out when the beam splitter apparatus is not rotating.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve anapparatus for beam splitting such that disturbing relative positionerrors of the sub-beams relative to one another are automaticallycorrected and such that an exact beam splitting is achieved.

According to the invention, an apparatus is provided for splitting alight beam into two sub-beams. The stationary light source generates alight beam along an optical axis. An objective is arranged in theoptical axis in front of the light source. A deflection unit is chargedby the light beam and is rotatable around the optical axis. Thedeflection unit has a polarization beam splitter for dividing thepolarized light beam into a reflected, polarized light component thatproceeds substantially perpendicular to the optical axis as a firstsub-beam and into a polarized light component that is allowed to passand proceed substantially in a direction of the optical axis. Thedeflection unit further has a polarization converter and a reflectorarranged on the optical axis behind the polarization beam splitter forturning the light component as it passes through the converter by 90° inphase with respect to the polarization direction and for reflecting thelight component back onto the polarization beam splitter where it isreflected at the polarization beam splitter to form a second sub-beam.Respective focus units are provided for the first and second sub-beams.In one embodiment of the invention for correcting relative positionoffsets of the first and second sub-beams relative to one another, thereflector comprises a prism with at least two mirror faces residing atan angle relative to one another for reflecting onto the polarizationconverter the light component passing through the polarization converterand into the prism. In another embodiment of the invention, the unit isprovided for determining relative positional offset of at least one ofthe sub-beams at a reference surface impinged on by the sub-beam and fordetermining correction values corresponding to the determined positionaloffsets. A unit is provided for changing an angle of inclination of atleast one of the polarization beam slitters relative to the optical axisor for shifting the focusing units for the sub-beams dependent on thecorrection values for correcting relative positional offsets of thesub-beams.

The invention is explained in greater detail below with reference toFIGS. 1 through 5.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an exemplary embodiment of a beam splitter apparatus;

FIG. 2 is an exemplary embodiment of a beam splitter apparatus with abeam position correction;

FIG. 3 is another exemplary embodiment of the beam splitter apparatuswith a beam position correction;

FIG. 4 is an applied example of the beam splitter apparatus; and

FIG. 5 is a development of the beam splitter apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an exemplary embodiment of a beam splitter apparatus thatis composed of a stationary light source 1 and of a deflection unit 2.The stationary light source 1 is, for example, a semiconductor laserdiode. The deflection unit 2 is seated in rotatable fashion around anoptical axis 3 and is driven by a motor 4. A light beam 5 that proceedsparallel to the optical axis 3 is generated in the stationary lightsource 1. The light beam 5 is divided in the deflection unit 2 into twosub-beams 6, 7 that proceed radially relative to the optical axis 3.

The beam splitting can be implemented with a non-polarized or with apolarized light beam 5. In the exemplary embodiment, the light source 1generates a linearly polarized light beam 5 that is converted into acircularly polarized light beam 5. Any other light source that generatesa non-polarized light beam can also be fundamentally used in conjunctionwith a polarizer to generate the linearly or circularly polarized lightbeam 5.

A lens system 8 and a first polarization converter 9 for thetransformation of a linear polarization into a circular polarization arearranged on the optical axis between the light source 1 and thedeflection unit 2. The first polarization converter 9 can be omittedwhen a non-polarized light beam 5 or a light beam 5 that is alreadycircularly polarized is utilized for the beam splitting.

In the illustrated exemplary embodiment, the lens system 8 generates aparallel, i.e. not pre-focussed light beam 5, and the polarizationconverter 9 is designed as a wave plate, for example as a λ/4 plate.

The deflection unit 2 is essentially composed of a polarization beamsplitter 12, of a second polarization converter 13 and of a reflector14. The optical elements are arranged following one another on theoptical axis 3 and are combined with one another, for example by gluing,to form a compact unit.

The polarization beam splitter 12 has a polarization layer 17 thateither reflects a polarized light beam or allows it to pass depending onthe polarization direction. The polarization layer 17 is usually formedof a plurality of dielectric layers with different refractive indices.Alternatively, the polarization layer 17 can be composed of a polarizingfilm of plastic or some other suitable material. A glass plate or, as inthe exemplary embodiment, a cube formed of two triangular prisms 18, 19can be employed as a carrier for the polarization layer 17, whereby thepolarization layer 17 is arranged in the region of those limitingsurfaces of the triangular prisms 18, 19 that face toward one another.

The second polarization converter 13 is, for example, a birefringentplate that, together with the reflector 14, effects a 90° rotation ofthe polarization plane of a linearly polarized light beam.

In order to avoid the involved adjustment of the light beam 5 emergingfrom the stationary light source 1 in the direction of the optical axis,as mentioned in the introduction to the specification, the reflector 14is designed according to the invention as a prism with at least twomirror faces residing at an angle of 90° relative to one another. Givensuch a prism, a light beam incident at an arbitrary angle in turnemerges from the prism parallel to the incident light beam deflected by180°. As shown in the exemplary embodiment, a ridge prism, or a tripleprism, or, triple mirror, can preferably be employed as the prism. Atriple prism comprises three mirror faces residing at 90° relative toone another that abut at a mirror corner. After three-fold reflection, alight beam incident into the triple prism at an arbitrary angle returnsparallel to the incident light beam deflected by 180°, whereby thedirectional reverse is invariant to a rotation of the triple prismaround an arbitrary axis.

The beam splitter apparatus comprises an optical unit for focussing thesub-beams 6, 7 onto a reference plane.

In the illustrated exemplary embodiment, wherein a light beam 5 that isnot pre-focussed is split, the first sub-beam 6 is focussed onto thereference plane by a lens 15 arranged in its beam path. Alternativelythereto, the lens 15 can also be arranged in the beam path of the lightbeam 5 between the lens system 8 and the deflection unit 2, for examplein stationary fashion behind the first polarization converter 9 or infront of the polarization beam splitter 12 in the rotating deflectionunit 2. The second sub-beam 7 is focussed onto the reference plane by alens 16 arranged in its beam path. Alternatively thereto, the lens 16can also be arranged in the rotating deflection unit 2 betweenpolarization beam splitter 12 and reflector 14. A lens combination canalso be employed instead of respectively one lens.

When, as in the illustrated exemplary embodiment, a light beam 5 that isnot pre-focussed is split, the lenses employed for the focussing of thefirst and second sub-beam 6, 7 have identical optical parameters. When,by contrast, a light beam that is pre-focussed by means of acorresponding design and arrangement of the lens system 8, thecorresponding lenses for focussing the first and second sub-beam 6, 7have different optical parameters, whereby the focussing unit for one ofthe sub-beams 6, 7 can be entirely eliminated in this case.

Having described the structure of the beam splitter apparatus, thefunctioning thereof shall be explained in greater detail.

The linearly polarized light beam 5 generated by the light source 1 isfirst collimated by the lens system 8 and is then converted into acircularly polarized light beam 5 in the first polarization converter 9.What the circular polarization of the light beam 5 achieves is that thepolarization effect of the polarization layer 17 of the polarizationbeam splitter 12 is independent of the respective rotational angle ofthe polarization beam splitter 12 or, of the rotating deflection unit 2.

The circularly polarized light beam 5 is incident onto the polarizationlayer 17 of the polarization beam splitter 12 and is divided thereatinto two linearly polarized light components, whereby the luminous powerof the light beam 5 is halved in nearly loss-free fashion. The firstlinearly polarized light component is reflected at the polarizationlayer 17 and forms the first sub-beam 6. When, as a result of an exactadjustment, the light beam 5 proceeds in the direction of the opticalaxis 3 and when the polarization layer 17 lies at an angle of 45°, thenthe first sub-beam 6 emerges from the deflection unit 2 perpendicular tothe optical axis 3 and reaches the reference plane in the ratedposition. When an adjustment is lacking or when an adjustment was notcarried out exactly, the light beam 5 is incident onto the polarizationlayer 17 at an angle relative to the optical axis 3. The first sub-beam6 emerges from the deflection unit 2 with a corresponding angulardeviation from the perpendicular and reaches the reference plane with apositional offset compared to the rated position.

The polarization layer 17 of the polarization beam splitter 12 allowsthe second linearly polarized light component 20 to pass and conducts itin the direction of the reflector 14 onto the second polarizationconverter 13 that modifies the polarization condition of the linearlypolarized light component 20. The light component 20' is reflected atthe reflector, whereby the polarization condition is again modified. Thereflected light component 20' again passes through the secondpolarization converter 13, but in the opposite direction. A rotation ofthe polarization plane by 90° relative to the polarization plane of thelight component 20 proceeding in the direction of the reflector 14 isthereby achieved, as a result whereof the light component 20' comingfrom the reflector 14 is reflected at the polarization layer 17 of thepolarization beam splitter 12 as second sub-beam 7.

When, due to an exact adjustment, the light beam again proceeds in thedirection of the optical axis 3, the second sub-beam 7 emerges from thedeflection unit 2 perpendicular to the optical axis 3 and, after half arevolution of the deflection unit 2, reaches the reference plane,likewise in the rated position on a line.

When, due to a lacking or imprecise adjustment, by contrast, the lightbeam 5 enters into the polarization beam splitter 12 inclined by anangle relative to the optical axis 3, the light component allowed topass is also incident onto the reflector 14 of the invention at thisangle and is cast back by this into itself, i.e. upon retention of therespective angle, onto the polarization converter 13 and thepolarization layer 17 of the polarization beam splitter 12. Although thesecond sub-beam 7 thereby still emerges from the deflection unit 2 withan angular deviation from the perpendicular, the angular deviation ofthe second sub-beam 7 has the same direction as the angular deviation ofthe first sub-beam 6 with reference to the rated position.

Due to the reflector 14 of the invention the angular errors now actidentically on both sub-beams 6, 7, and the disturbing influence of thepositional offset of the two sub-beams 6, 7 on, for example, therecording quality of a recorder is advantageously diminished.

Over and above this, the employment of the reflector (14) of theinvention has the advantage that position tolerances that arise when thereflector is built into the deflection unit 2 are not critical withreference to the positional offset of the sub-beams 6, 7, by contrast tothose of a plane mirror.

For complete correction of the positional offset of the two sub-beams 6,7 and for simultaneous correction of potentially existing integrationtolerances, it is additionally proposed that an automatic beam positioncorrection be implemented in the rotating light deflector such that bothsub-beams 6, 7 are always located in the rated position in the referenceplane onto which both sub-beams 6, 7 are focussed.

In, for example, a recorder, a good recording quality is therebyachieved independently of angular errors of the light beam 5 incidentonto the deflection unit 2 and of integration tolerances of thepolarization beam splitter 12 i.e. without involved manual adjustments.

The automatic beam position correction advantageously occurs bymodification of the angle of inclination of the polarization layer 17 ofthe polarization beam splitter 12 and/or by shifting at least one of thelenses 15, 16 essentially in the direction of the optical axis 3.

The modification of the angle of inclination of the polarization layer17 of the polarization beam splitter 12 is achieved by tilting thepolarization beam splitter or by tilting the entire deflection unit 2when polarization beam splitter 12, polarization converter 13 andreflector 14 are compactly combined with one another to form a unit.

FIG. 2 shows an exemplary embodiment of a beam splitter apparatuswherein polarization beam splitter 12, polarization converter (13) andreflector 14 are compactly joined to one another, and whereby a tiltangle control of the entire rotating deflection unit 2 is implementedfor the automatic beam position correction, so that a change of theangle of inclination of the polarization layer 17 and a shift of thelenses 15, 16 occur simultaneously. As a result of the automatic beamposition correction, the reflector 14 can be designed as a plane mirroror, as shown, as a triangle prism or, respectively, as a triple prism.

With a bearing 21, the entire deflection unit 2 is seated at acylindrical carrier 22 tiltable around the optical axis 3, An adjustmentdrive 23 for tilting the deflection unit is built into the carrier 22,as is the winding 24a of at least one transformer 24 whose other winding24b is stationarily arranged at a distance from the carrier 22. Themechanical structures of bearing 21, carrier 22 and adjustment drive 23must be designed such that a rotational-symmetrical mass distribution isachieved. For example, the adjustment drive 23 is a piezo drive or asuitable magnetostrictive drive. Such drives are commerciallyobtainable. The transformer 24, for example, is an electromagnetictransformer with which an energy transmission for the piezo drive, whichrequires less than 1 mW power, can be realized without further ado.

The acquisition of the positional offsets of the two sub-beams 6, 7relative to one another and the determination of appropriate correctionvalues for the beam position correction can occur in a variety of ways.

The positional offsets of the sub-beams 6, 7 relative to one another inthe reference plane (focus plane) can, for example, be measured with aposition-measuring unit 25, as shown in the exemplary embodiment. Forexample, the position-measuring unit 25 is designed as a differentialphotodiode with two light-sensitive surfaces separated from one another.The boundary line between the light-sensitive surfaces of thedifferential photodiode extends in the deflection direction of thesub-beams 6, 7 and is located in the rated position for the twosub-beams 6, 7. The photocurrents of the differential photodiodegenerated by the incident sub-beams 6, 7 are supplied to a comparisonstage 26 that generates positional error values as a criterion for thepositional offset of the sub-beams 6, 7 perpendicular to the deflectiondirection. Such a position-measuring unit is disclosed in detail in, forexample, EP-B-0 054 170.

The positional error values are supplied to a servo amplifier 27 that isconnected to the stationary winding 24a of the transformer 24. Thepositional error values are converted into corresponding correctionvalues in the servo amplifier that determine the needed direction andamplitude of the adjustment of the adjustment drive 23. For energytransmission to the adjustment drive 23, a primary alternating voltageis generated from the correction values and this is supplied to thestationary winding 24a. The primary alternating voltage induces asecondary alternating voltage in the rotating winding 24b of thetransformer that is converted into a d.c. voltage in a rectifier. Thed.c. voltage supplied to the adjustment drive 23 effects a correspondingtilting of the deflection unit 2 until a tilt angle is reached at whichthe two sub-beams 6, 7 are located in the rated position in thereference plane.

The correction values needed for the correction of the positional offsetof the sub-beams 6, 7 are determined, for example, in a measuring phasepreceding the actual operation and are stored in the servo amplifier 27.During operation, the stored correction values are then transmitted tothe adjustment drive 23 for ongoing position correction of the twosub-beams 6, 7.

The measurement of the positional offset of the sub-beams 6, 7 with theposition-measuring unit is undertaken at a separate measurement locationfor the beam splitter apparatus, but preferably in the same device intowhich the beam-splitter apparatus is built. When the beam splitterapparatus is employed in an exposer or recorder, the reference surfacefor measuring the positional offset is the exposure plane on which thefilm material to be exposed is arranged. In this case, the positionerror values or, correction values for the beam position correction canalso be alternatively determined for measuring the positional error suchthat a film first is exposed stripe-like in the recorder (exposer) withdifferent, predetermined correction values, and the exposed film isvisually or mensurationally interpreted for determining the optimumcorrection value.

It is basically adequate to provide only one transformer 24 at thecircumference of the carrier 22 and to transmit only one correctionvalue per revolution to the piezo drive since this has a capacitativebehavior. Due to the rotational-symmetrical mass distribution, however,it is expedient to provide a plurality of transformers 24 or, aplurality of windings 24b rotating with the carrier 22 and stationarywindings 24a.

If a simultaneous shift of the lenses 16, 17 should not occur given thetilting of the deflection unit 2, the deflection unit 2 of FIG. 2 can becorrespondingly structurally modified. For that purpose the lenses 15,16 can be seated, for example, in a cylindrical lens mount open at oneside that envelopes the components of the deflection unit 2 and whoseone end face is secured to the outside surface of the carrier 22, sothat the lens mount does not follow the tilt motion of the deflectionunit 2.

It lies within the framework of the invention to correct the positionaloffsets of the sub-beams 6, 7 with an ongoing control during operationin that the positional offsets are continuously measured andcorresponding correction values are transmitted to the rotatingdeflector 2.

FIG. 3 shows a further exemplary embodiment of a beam splitter apparatuswherein the automatic beam position correction is implemented byshifting the lenses 15, 16 in the direction of the optical axis 3 withthe adjustment drives 23. In this embodiment, too, the reflector 14 canbe designed as a plane mirror or, as shown, as a triangle prism ortriple prism. The measurement of the positional offsets of the sub-beams6, 7 and the transmission of the corresponding correction values to theadjustment drives occurs in the way described in FIG. 2.

FIG. 4 shows an applied example of the inventive beam splitter apparatusof the invention in a scanner unit 28 of a recorder exposer workingaccording to the inside drum principle. In such an inside drum recorder,the recording material 29 is fixed to the inside wall of a cylindersegment as an exposure trough 30. The scanner unit 28 rotates around thelongitudinal axis 31 of the exposure trough 30. The scanner unit 28comprises the beam splitter apparatus shown in FIG. 1. The sub-beams 6,7 are brightness-modulated by an image signal and are conducted over therecording material 29 point-by-point and line-by-line for exposure. Thescanner unit 28 thereby moves in the direction of the longitudinal axis31 on the basis of a drive not shown. Each revolution of the light beamdeflection unit is divided into a working region 32 or working timeinterval and a return region 33 or a return time interval. While the onesub-beam 6 or 7 is active and sweeps the working region 32 from thestart of a line to the end of the line for exposing the recordingmaterial 29, the inactive sub-beam 7 or 6 is returned in the returnregion 33 to the start of the next line. The degree of utilization ofthe exposer is doubled by the recording with two sub-beams 6, 7.

In the beam splitter apparatus described up to now for employment inexposers, the two exposing sub-beams 6, 7 each have respectively 50% ofthe luminous power of the input light beam 5 supplied to the beamsplitter apparatus. However, a higher luminous power is often requiredfor exposing specific materials. In order to achieve a higher luminouspower, it is proposed according to the invention that the creation ofthe respectively inactive sub-beam 6 or, 7 be prevented during thereturn interval, so that no division of the luminous power ensues andthe active sub-beam 7 or, 6 comprises nearly the full luminous power ofthe input light beam 5.

FIG. 5 shows an advantageous development of a beam splitter apparatuswith an inactive sub-beam 6 or, 7 that can be shut off in the respectivereturn interval for increasing the luminous power of the respectivelyactive sub-beam 7 or, 6.

The beam splitter apparatus of FIG. 5 differs from that according toFIG. 1 in that the stationary, first polarization converter 9 isreplaced by a controllable polarization converter 34, and in that apolarization converter 35 that can be connected to the polarization beamsplitter 12, for example by gluing, is arranged in the rotatingdeflection unit 2 preceding the polarization beam splitter 12. Further,a programmable angular momentum generator 36 that is mechanicallyconnected to the motor 4 and supplies a two-level control signal to thecontrollable polarization converter 34 via a line 37 is arranged on theoptical axis.

The angular momentum generator 36 is programmable such that, in everyrevolution of the deflection unit 2, the two-level control signalassumes the one signal level during the working region (FIG. 4; 32) ofthe one sub-beam 6 or, 7 and assumes the other signal level during thereturn region (FIG. 4; 33) of the other sub-beam 7 or 6. The lengths ofthe work regions and of the return regions of the sub-beams 6, 7 canthus be advantageously defined independently of one other by the type ofprogramming.

The controllable polarization converter 34 contains an electro opticalmodulator that converts the linear polarization of an incident lightbeam into a circular polarization and switches the rotational sense ofthe circular polarization dependent on the respective signal level ofthe control signal on the line 37. The polarization converter 34 isfashioned as wave plate, for example as a λ/4 plate.

The linear polarization of the light beam 5 coming from the light source1 is first converted into a circular polarization in the controllablepolarization converter 34. The rotational sense of the circularpolarization is then switched dependent on the respective signal levelof the control signal. The conversion of the linear polarization of thelight beam 5 into the circular polarization and the change of therotational sense of the circular polarization can, of course, also beundertaken in separate assemblies.

In the polarization converter 35 rotating with the deflection unit 2,the circular polarization of the light beam 5 is converted--depending onthe respective rotational sense of the circular polarization--into alinear polarization with a first polarization condition, for example alinear polarization with a 0° polarization plane, or with a secondpolarization condition, for example into a linear polarization with a90° polarization plane.

When the linearly polarized light beam 5 exhibits the first polarizationcondition, the light beam 5 is reflected at the polarization layer 17 ofthe polarization beam splitter 12, but is not allowed to passtherethrough, so that only the first sub-beam 6 arises with nearly thefull luminous power of the light beam 5. When, by contrast, the lightbeam 5 exhibits the second polarization condition, the light beam 5 isallowed to pass by the polarization layer 17 of the polarization beamsplitter 12 but is not reflected thereat, so that only the secondsub-beam 7 is formed with nearly the full luminous power of the lightbeam 5.

The beam splitter apparatus of the invention can be employed inrecording devices of the inside drum type or of the flatbed type. Italso lies within the scope of the invention to utilize the beam splitterapparatus in originals scanner means.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that my wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within my contribution tothe art.

I claim:
 1. An apparatus for splitting a light beam into a first and asecond sub-beam, comprising:a stationary light source for generating alight beam on an optical axis; an objective arranged in the optical axisin front of the light source; a deflection unit rotatable about theoptical axis, said light beam being incident onto said deflection unit;a polarization beam splitter in said deflection unit arranged on theoptical axis for dividing the light beam into a first polarized lightcomponent as said first sub-beam reflected by said polarization beamsplitter substantially perpendicular to the optical axis and into asecond polarized light component transmitted by said polarization beamsplitter substantially in a direction of the optical axis, apolarization converter and a reflector being arranged on the opticalaxis behind said polarization beam splitter for phase turning the secondlight component by 90° in phase with respect to a polarization directionand for reflecting the phase-turned second beam component back to thepolarization beam splitter at which the phase-turned second beamcomponent is reflected as said second sub beam substantiallyperpendicular to said optical axis; respective focusing units in saiddeflection unit for focusing said first and second sub-beams ontorespective surfaces; for correcting relative positional offsets of thefirst and second sub-beams relative to one another, the reflectorcomprising a prism with at least two mirror faces residing at an anglerelative to one another for reflecting the phase-turned second beamcomponent back to said polarization converter after the second beamcomponent has passed through the polarization converter to said prism.2. An apparatus according to claim 1 wherein the two mirror faces resideat an angle of 90° relative to one another.
 3. The apparatus accordingto claim 1 wherein the prism is designed as a triangle prism.
 4. Theapparatus according to claim 3 wherein at least two mirror faces of thetriangle prism are at an angle of 90° to one another.
 5. The apparatusaccording to claim 1 wherein the prism is designed as a triple prism. 6.The apparatus according to claim 5 wherein at least two mirror faces ofthe triple prism are at 90° relative to one another.
 7. The apparatusaccording to claim 1 wherein the focusing unit for the first sub-beamcomprises at least one lens in the beam path of the first sub-beam. 8.The apparatus according to claim 1 wherein the focusing unit for thesecond sub-beam comprises at least one lens in the beam path of thesecond sub-beam.
 9. The apparatus according to claim 1, furthercomprising:means for determining a relative positional offset of atleast one of the sub-beams at a reference surface impinged by thesub-beam and for a determining a correction value corresponding to thedetermined positional offset; means for transmission of the correctionvalue to the deflection unit; and means for changing an angle ofinclination of the polarization beam splitter of the deflection unitrelative to the optical axis depending on the transmitted correctionvalue for correcting relative positional offsets of the sub-beams. 10.The apparatus according to claim 9, wherein:the angle of inclination ofthe polarization beam splitter is changeable by tilting the deflectionunit relative to the optical axis; and the means for changing the angleof inclination of the polarization beam splitter is an adjustment drivewhich is in an interactive connection with the deflection unit andcontrollable by said transmitted correction value for tilting saiddeflection unit.
 11. The apparatus according to claim 10, wherein theadjustment drive comprises a piezo drive.
 12. The apparatus according toclaim 10 wherein the adjustment drive comprises a magnetostrictivedrive.
 13. The apparatus according to claim 9 wherein the means fordetermining the relative positional offset of the at least one sub-beamand for determining the correction value corresponding to the determinedpositional offset comprises a position-measuring unit arranged at saidreference surface struck by the sub-beam.
 14. The apparatus according toclaim 13 wherein a differential photo diode is employed as theposition-measuring unit.
 15. The apparatus according to claim 9 whereinthe means for the transmission of the correction value to the deflectionunit comprises an electro-magnetic transformer.
 16. The apparatusaccording to claim 1 further comprising:means for determining a relativepositional offset of at least one of the sub-beams at a referencesurface impinged by the sub-beam and for determining a correction valuecorresponding to the determined positional offset; means fortransmission of the correction value to the deflection unit; and meansfor shifting a position of said at least one focusing unit depending onthe transmitted correction value for correcting relative positionaloffsets of the sub-beams.
 17. The apparatus according to claim 16wherein the means for changing the angle of inclination of the focusingunits comprises an adjustment drive at each focusing unit controllableby the correction value.
 18. The apparatus according to claim 17 whereinthe adjustment drive comprises a piezo drive.
 19. The apparatusaccording to claim 17 wherein the adjustment drive comprises amagnetostrictive drive.
 20. The apparatus according to claim 16 whereinthe means for determining the relative positional offset of the at leastone sub-beam and for determining the correction value corresponding tothe determined positional offset comprises a position-measuring unitarranged at the reference surface of the sub-beam.
 21. The apparatusaccording to claim 20 wherein a differential photo diode is employed asthe position-measuring unit.
 22. The apparatus according to claim 16wherein the means for transmission of the correction value to thedeflection unit comprises an electro-magnetic transformer.
 23. Theapparatus according to claim 1 wherein the light source generates acircularly polarized light beam.
 24. The apparatus according to claim 1wherein:the light source generates a linearly polarized light beam; anda further polarization converter for converting the linear polarizationof the light beam into a circular polarization is arranged on theoptical axis between the light source and the rotating deflection unit.25. The apparatus according to claim 24 wherein the polarizationconverter and the further polarization converter are designed asbirefringent plates.
 26. The apparatus according to claim 23 wherein thebirefringent plates are λ/4 plates.
 27. The apparatus according to claim1 for increasing luminous power of the respectively active first andsecond sub-beams further comprising:a further polarization converterstationarily arranged in the optical axis in front of the deflectionunit and controllable by a control signal for switching over arotational direction of a circular polarization of a circular polarizedlight beam depending on said control signal; another polarizationconverter arranged in the optical axis and on the deflection unit infront of the polarization beam splitter for converting said circularpolarized light beam into a linear polarized light beam having a firstpolarization plane or a second polarization plane depending on arespective rotational direction of the circular polarization of thecircular polarized light beam; the converted linear polarized light beamhaving said first polarization plane is reflected by the polarizationbeam splitter as said first polarized light component for generatingsaid active first sub-beam, but is not transmitted as said secondpolarized light component to generate said second sub-beam; and theconverted linear polarized light beam having said second polarizationplane is transmitted through the polarization beam splitter as saidsecond polarized light component for generating said active secondsub-beam, but is not reflected as said first polarized light componentto generate said first sub-beam.
 28. The apparatus according to claim 27wherein the controllable further polarization converter is designed asan electro-optical modulator.
 29. The apparatus according to claim 27wherein the further converter is designed as a wave plate.
 30. Theapparatus according to claim 27 wherein an angular momentum generator iscoupled with the deflection unit for generating the control signal forthe further polarization converter, the control signal determining aworking interval in which one of the sub-beams is activated and theother sub-beam is de-activated in every revolution of the deflectionunit.
 31. An apparatus for splitting a light beam into first and secondsub-beams, comprising:a light source for generating a light beam alongan optical axis; an objective arranged on the optical axis in front ofthe light source; a deflection unit receiving the light beam passingthrough the objective and rotatable around the optical axis, saiddeflection unit having:a polarization beam splitter for dividing thepolarized light beam into a reflective, polarized light component assaid first sub-beam that proceeds substantially perpendicular to theoptical axis, and into a polarized light component that is allowed topass and proceeds substantially in a direction of the optical axis; apolarization converter and a reflector after the polarization converterarranged on the optical axis behind the polarization beam splitter, thepolarization converter turning the light component as it passes throughthe converter by 90° in phase with respect to the polarizationdirection, and the reflector reflecting the light component passingthrough the polarization converter back through the polarizationconverter and to the polarization beam splitter where it is reflected atthe polarization beam splitter to form said second sub-beam; and thereflector comprising a three sided triangular prism having a facerunning perpendicular to the optical axis and facing the polarizationconverter, said reflector being designed such that the light beamincident at an arbitrary angle in turn emerges from the prism parallelto the incident light beam and deflected by 180°.
 32. The apparatusaccording to claim 31, wherein said triangular prism has at least twomirror faces at 90° relative to one another.
 33. An apparatus forsplitting a light beam into first and second sub-beams, comprising:alight source for generating a light beam along an optical axis; anobjective arranged on the optical axis in front of the light source; adeflection unit receiving the light beam passing through the objectiveand rotatable around the optical axis, said deflection unit having:apolarization beam splitter for dividing the polarized light beam into areflective, polarized light component as said first sub-beam thatproceeds substantially perpendicular to the optical axis, and into apolarized light component that is allowed to pass and proceedssubstantially in a direction of the optical axis; a polarizationconverter and a reflector arranged on the optical axis behind thepolarization beam splitter, the polarization converter turning the lightcomponent as it passes through the converter by 90° in phase withrespect to the polarization direction, and the reflector reflecting thelight component passing through the polarization converter back throughthe polarization converter and to the polarization beam splitter whereit is reflected at the polarization beam splitter to form said secondsub-beam; respective focusing units for the first and second sub-beams;a position measuring unit for determining a relative positional offsetof at least one of the sub-beams on a reference surface and fordetermining a correction value corresponding to the determinedpositional offset; and a unit for changing at least one of angle ofinclination of the polarization beam splitter relative to the opticalaxis or position of the focusing units for the sub-beams dependent onthe correction value for correcting relative positional offset of thesub-beams.
 34. The apparatus according to claim 33 wherein the reflectorhas at least two mirror faces arranged at 90° relative to one another.